Scientists have defended the use of rapid coronavirus tests, saying they could act as “early warning systems” for local outbreaks ahead of a mass rollout by the government.

Lateral flow tests (LFTs) have been used as part of mass testing schemes in Liverpool and Birmingham, but recent reports say they can be inaccurate.

A group of scientists now believe, however, that disparities between the number of positive results showing up on rapid tests and lab-tested swabs could reveal whether a local outbreak is growing or shrinking.

It comes as the government is reportedly preparing a new ‘Are you ready? get testing, go’ scheme that would see three million rapid tests handed out nationwide each week to help unlock parts of the economy.

The new commentary published in the Lancet says that if more people are testing positive by PCR (polymerase chain reaction) tests than they are by LFTs then cases in that area are going down.

But if more infections are being detected through LFTs than PCR tests, cases are going up.

“Measuring LFT results can assist as an early warning system that infections are increasing as its use offers more windows on an area’s epidemic curve,” the research says.

The reason high numbers of PCR-positives could mean cases are declining is that between 50 and 75% of people who test positive using a PCR test are no longer infectious, the researchers say.

This is because these tests can detect extremely small levels of the virus that “can linger weeks after infectious virus has cleared”.

By contrast, LFTs can only show if you are infectious on that particular day and are less likely to detect leftover molecules from an old infection.

But other scientists have expressed their “despair” over the government’s plans to use lateral flow to help ease lockdown.

Jon Deeks, professor of biostatistics at Birmingham University, said “There are so many things wrong with this approach”, adding: “There is no evidence for using these tests as green light tests. Despair.”

Public health specialist Dr Angela Raffle told Sky News the new analysis “sidesteps a lot of the really serious issues” around testing.

“There’s a fundamental difference between trying to pick up asymptomatic cases versus telling someone they are safe to go and do something,” she said.

“And this ignores a lot of what is really concerning scientists at the moment.”

Dr Raffle expressed her concern about the lack of mention of five key issues: how testing is delivered; whether people are properly trained to do it; if self-testing is reliable; whether other types of test may prove more valuable; and the need to support people if they need to self-isolate.

Robert Dingwall, a professor of sociology on the government’s SAGE committee, told Sky News LFTs are “pretty useless”.

“These mass tests miss people with early infections, so they are not really any kind of solution,” he said.

“The government has bought a huge stockpile of these tests and something has to be done with them,” he added.

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The new analysis has been published in response to data that showed LFT pilot schemes in Liverpool and Birmingham only had 66% and 3% rates of sensitivity respectively.

Researchers who worked on the Lancet analysis say that the low levels of LFT sensitivity meant that both outbreaks were on the decline – not that the testing was inaccurate.

Iain Buchan, professor of public health and clinical informatics at the University of Liverpool, is one of five scientists who worked on it.

He said: “People with the virus can only usually pass it onto others for a short period, around four to eight days early in their infection, after which they will not be infectious but still test PCR-positive for a couple of weeks or more, as PCR picks up genetic material left behind after the body has dealt with the virus.

“Lateral flow on the other hand is designed to pick up material from infectious virus coming out of the noses and throats of people in that short period early on.

“So, when an epidemic is past its peak, and numbers of new infections are falling relative to old infections, lateral flow will look less sensitive if compared with PCR.”

He said that despite “confusion” over LFT inaccuracy: “Lateral flow tests can play an important part in making places where people mix, at work or socially, safer as lockdown eases, provided they are used properly.”

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